Hey armorchris,
Well, nearly every Ferdinand in the German inventory was sent to the Eastern Front just prior to the Battle of Kursk. Almost all of them were destroyed or captured in the battle. Those that survived, were upgraded into Elefants before redeployment. Although the official order for three-color camo was issued in February 1943, many units did not adopt the practice until late summer. Additionally, the order gave local commanders the widest latitude to customize vehicle paint schemes for the local terrain. Therefore there was a great diversity of paint schemes on German armor in 1943 Russia.
Based on this, your base color will be an overall coat of MM Sandgelb RLM 79. You could leave the vehicle like this (weathered of course), go to a two-color camo scheme, or go all the way to a three-color scheme. It really depends on the unit you are depicting with your model.
If you go with two-color, use olive green over the yellow in either stripes or blotches in various designs. MM Panzer Olivgrun 1943 is the paint of choice here and doesn't look too bad as is for 1/35 scale. If you build 1/72, you might want to lighten it a bit.
Finally, if you go with a three-color scheme, MM Schokoladenbraun '43 is the color that they claim is the proper third color. I've found that this color is quite dark and usually lighten it in varying amounts depending on the model. Many modelers have stated that they use slightly darkened MM Leather, darkened MM Rust, or other combinations of colors for their version of the right "red brown RAL 8017".
I hope this helps out, but don't hesitate to ask additional questions if you want.
Enjoy your modeling...